Case Review: Accessory Spleen Adjacent to the Tail of the Pancreas

Accessory spleens occur in approximately 10–30% of individuals, most commonly near the splenic hilum. A smaller but clinically significant subset lies adjacent to the tail of the pancreas, where their vascular enhancement closely resembles that of solid pancreatic tumours. Misinterpretation of these lesions has led to multiple reports of inadvertent resection during surgery for suspected neoplasm. This case review synthesises all available data on accessory spleens located at or near the pancreatic tail, integrating radiological cohorts, surgical and pathological series, and individual case reports to clarify prevalence, imaging behaviour and clinical risk.

A structured search of PubMed, Scopus and Web of Science was conducted using the terms “accessory spleen,” “intrapancreatic accessory spleen,” “pancreatic tail” and “splenule.” Eligible publications included radiological cohorts (CT, MRI, MDCT angiography), surgical and pathological series, and case reports or case series describing accessory spleens located at, within or immediately adjacent to the pancreatic tail. For each study, the following variables were extracted where available: total sample size, number and proportion of accessory spleens, proportion specifically at the pancreatic tail, lesion size, enhancement characteristics, diagnostic modality, initial working diagnosis and operative outcome. Because of heterogeneity in reporting and the case-focused nature of many articles, data were synthesised descriptively rather than by formal quantitative meta-analysis.

Large MDCT and MRI cohorts demonstrate that accessory splenic tissue at the pancreatic tail is not an exceptional finding. Across combined datasets exceeding 12,000 patients, accessory spleens at or within the pancreatic tail were reported in roughly 1.0–3.0% of the general population and accounted for 11–17% of all detected accessory spleens.^{1, 2} Typical lesions measured between 0.5 and 3.0 cm and were supplied by branches of the splenic artery, with enhancement patterns that closely paralleled those of the native spleen. Recognition of this predictable behaviour is central to distinguishing these nodules from small pancreatic-tail neoplasms.

In more than 80 published case reports and small series, accessory spleens at the pancreatic tail were most frequently mistaken for pancreatic neuroendocrine tumours, followed by solid pseudopapillary tumours, metastatic deposits and, less commonly, ductal adenocarcinoma.^{4, 5} Misdiagnosis was particularly likely when lesions exceeded 1.5 cm, when dual-phase imaging was not performed, or when enhancement was assessed only qualitatively. In a substantial subset of cases, the presumed neoplasm prompted distal pancreatectomy with or without splenectomy, only for histopathology to reveal benign accessory splenic tissue. Conversely, failure to recognise accessory spleens during splenectomy for haematologic disease can leave functioning splenic tissue in situ, undermining the therapeutic intent of the operation.

Embryologically, accessory spleens arise when splenic tissue buds along the dorsal mesogastrium fail to coalesce into a single organ. When these foci persist near the tail of the pancreas, their arterial supply and parenchymal composition mirror those of the main spleen, explaining why they behave as perfect imaging mimics of small pancreatic tumours. On multiphase CT, accessory spleens at the pancreatic tail show arterial and portal venous phase enhancement that is isoattenuating to the spleen, and on MRI they exhibit similar T1, T2 and diffusion characteristics. Tc-99m heat-damaged red blood cell scintigraphy remains the most specific modality when available, confirming splenic tissue through selective tracer uptake.^{1, 7}

Clinically, these lesions matter in two opposing scenarios. First, in patients investigated for suspected pancreatic neoplasm, failure to consider accessory spleen in the differential diagnosis may lead to unwarranted distal pancreatectomy. Second, in splenectomy for haematologic disorders, unrecognised accessory spleens can remain functional within the abdominal cavity, contributing to persistent or recurrent disease.^{6, 8} Taken together, the case literature indicates that accessory spleens at the pancreatic tail should be regarded as predictable variants rather than exotic anomalies.

Accessory spleens adjacent to the pancreatic tail represent a stable, clinically relevant anatomical variant with a prevalence in the low single-digit percentage range. Their characteristic enhancement pattern, closely paralleling that of the main spleen, and their predilection for the pancreatic tail make them a key mimic of small pancreatic masses. A case-based understanding of their typical imaging features, prevalence and diagnostic pitfalls can prevent unnecessary distal pancreatectomy and improve the planning of splenectomy in haematologic disease. Rather than being dismissed as rare curiosities, pancreatic-tail accessory spleens should be integrated into routine diagnostic algorithms for pancreatic-tail lesions.